JSON and XML are both widely used formats for structured data exchange, and both solve the same fundamental problem: representing hierarchical data in a way that humans can read and machines can parse. But they make different trade-offs, and knowing when to reach for each one matters more than the format war that occupied many blog posts a decade ago.

The syntax difference

JSON uses curly braces for objects, square brackets for arrays, and colons to separate keys from values. It is a subset of JavaScript syntax and maps directly to the data structures of most modern programming languages.

XML uses tags — an opening tag, content, and a closing tag — much like HTML. Attributes can live inside the opening tag; child data lives between the tags. Everything must be wrapped in a single root element.

In practice, the same data takes significantly more characters to express in XML than in JSON. A list of three users in JSON might be 200 characters; the same data in XML with equivalent fidelity might be 400+ characters. That verbosity was a genuine problem when bandwidth was expensive, and it is part of why JSON displaced XML for most web API use cases in the 2010s.

Where JSON wins

• Web APIs and REST services. JSON parses directly into native objects in JavaScript, Python, Ruby, Go, and most other languages used for web development. No special XML parser library needed.
• Configuration files. JSON (and its supersets like JSONC or JSON5) is used for package.json, tsconfig.json, VS Code settings, and countless other tool configs.
• NoSQL databases. MongoDB, CouchDB, DynamoDB, and similar databases store documents in JSON (or BSON, a binary encoding of JSON).
• Lightweight data exchange. For simple key-value structures or arrays, JSON is shorter, faster to parse, and easier to debug.

Where XML still wins

• Document-centric content. XML was designed for documents, not data. If your content is prose with mixed inline markup — like <em>, <strong>, and <footnote> tags interspersed in text — XML handles it naturally. JSON does not have a good story for mixed content.
• Enterprise and legacy systems. SOAP web services, EDI formats like HL7 in healthcare, and many government data exchanges still use XML. If you are integrating with these systems, you work with what they give you.
• Schemas and namespaces. XML has a mature ecosystem for schema validation (XSD) and namespaces. These features matter in contexts where you need formal contracts between systems, especially cross-organisation.
• Transformations. XSLT is a powerful (if verbose) language for transforming XML into other XML, HTML, or plain text. Nothing in the JSON world is quite equivalent.

Converting between JSON and XML

Converting between the two is straightforward for simple structures, but has edge cases for complex ones. The main challenges:

• XML attributes have no direct JSON equivalent. A common convention is to map them to a key prefixed with @ or stored in a _attributes sub-object.
• JSON arrays map cleanly to repeated XML elements, but the element name has to come from somewhere — usually the parent key with the 's' dropped, or a convention in the conversion tool.
• JSON null values have no direct XML representation. They are usually omitted or represented as an empty tag.

For one-off conversions, a browser tool is the fastest option. For automated pipelines, use a library that documents exactly which conventions it follows — consistency matters more than which convention you pick.

The short answer

Use JSON for new web APIs, configuration, and data interchange between modern systems. Use XML when you are working with legacy systems, document-centric content, or a domain (healthcare, finance, government) that has standardised on XML schemas. And when you need to move data between the two, a reliable converter will handle the structural mapping for you.